The present invention relates to optical measurement devices, and more particularly to a twisting fiber depolarizer for overcoming polarization-dependent loss (PDL) in optical components of such optical measurement devices.
Many optical components exhibit polarization-dependent loss (PDL). Dichroic polarizers, such as used in sunglasses, are one example. Other examples are optical circulators, couplers and diffraction gratings. The existence of PDL in optical test instruments results in errors in optical power measurements. The reason is that the state of polarization of light being tested is uncontrolled, i.e., may be in any state, yet the existence of PDL ensures that the optical power measured by such instruments as an optical power meter, optical time domain reflectometer (OTDR) or optical spectrum analyzer (OSA) is a function of the state of polarization. Therefore if the input state of polarization happens to be in one particular configuration, the instrument returns a different power measurement than if the input polarization is in a different state.
PDL in OTDRs results in waveform noise that cannot be averaged away and is both aesthetically distracting as well as disruptive to both automatic and manual measurements of waveform events. In one situation PDL of less than 0.1 dB resulted in very objectionable waveform degradation, which illustrates the severity of the problem with PDL even when present in seemingly trivial amounts. Strongly polarized light sources are especially common in wave division multiplexed (WDM) systems, and low PDL is a key specification touted by manufacturers of OSAs. The problem with OSAs may be especially acute since many dispersive components in OSAs exhibit high PDL, i.e., the diffraction efficiency of a diffraction grating may be roughly 80% for one state of polarization and 20% for the orthogonal state.
Because of the importance of low PDL and the difficulty associated with designing it out of optical components in an instrument, it is essential that the input state of polarization be scrambled, i.e., depolarized. The job of the depolarizer is to make sure that even if the light coming into the test equipment is 100% polarized, it is depolarized before entering the OTDR or OSA and going through any optical components with high PDL. For most purposes a depolarized state may be taken as one in which the transmission of the light through a linear polarizer is independent of the angular orientation of the polarizer. For example, if a person goes outside on a sunny day with a pair of polarized sunglasses and looks at the painted side of a building while rotating the glasses, a variation in the apparent brightness of the building is not observed. The paint of the building reflects unpolarized light. Looking at the sky and rotating the glasses, on the other hand, produces a change in the brightness of the sky in a cyclical manner since the sky is partially polarized. The same phenomenon is observed when looking through polarized sunglasses at light that""s been reflected from a shiny surface.
A class of active depolarizers work by passing light through a time-varying birefringent medium. One such single path approach is to have two devices for introducing birefringence in an optical fiber. These devices may be piezoelectric transducers or electromagnetic devices. When the fiber is pinched by a first pincher, the stress causes the fiber to become birefringent, setting up a fast and slow axis in the cross-sectional plane of the optical fiber. A second pincher follows oriented 45 degrees relative to the first. If the stress applied by the pinchers varies with time, the state of polarization emanating from the fiber changes with time. Averaging over a sufficiently long period of time produces an apparent random state of polarization. Problems arise with accurate phase and amplitude control, minor variations of which may result in relatively poor polarization scrambling. Another problem is that the force exerted on the fiber by the pinchers may eventually damage the fiber.
Polarization-maintaining fiber, called HiBi fiber, has birefringence built into the fiber""s core. Birefringence means that the fiber""s core has an index of refraction that depends upon the light""s state of polarization. Light polarized in the vertical direction, for example, xe2x80x9cseesxe2x80x9d a different index of refraction than light polarized in the horizontal direction. The difference in the index of refraction between these two eigenmodes is the birefringence. FIG. 1 shows an end view of an HiBi fiber having a core, a cladding and stress members. The stress members serve to establish birefringence in the optical fiber, resulting in orthogonal axes. Light polarized along a vertical eigenmode experiences a different index of refraction than light polarized along a horizontal eigenmode, as discussed above. HiBi fiber has the property that, once polarized light is injected into the fiber, the state of polarization with respect to the two eigenmodes is maintained. In other words as the eigenmodes are rotated by twisting a chunk of HiBi fiber, the polarized light in the fiber twists right along with it, as shown in FIG. 2. The round circles with orthogonal black lines represent the two eigenmodes of the HiBi fiber. The line with the arrows at each end represents the state of polarization of the light within the fiber which rotates along with the twisting of the HiBi fiber.
What is desired is a simple fiber depolarizer to minimize the effect of PDL in optical components.
Accordingly the present invention provides a twisting fiber depolarizer having a fiber assembly of a nominally non-birefringent fiber spliced between two birefringent (HiBi) fibers, with the HiBi fibers held rigidly at two pointsxe2x80x94one point being one of the splice points and the other point being a point on the HiBi fiber on the opposite side of the other splice point. A torsion is applied to the free portion of the fiber assembly between the hold points using a back and forth motion. The twisting of the free portion of the fiber assembly over a time interval serves to randomize the polarization of light applied to one end of the fiber assembly when it appears at the other end.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.